the higher your tank the more lighting you will need id stick to 450mm for height especially if you use t5s it will mean you can keep more light demanding corals in the future. some good beginner corals are kenya tree, mushrooms, xenia, green star poylps, leathers, even some lps like torch and hammer corals arent too bad for beginners. with regards to fish some are quite hardy likes clowns , cardinals and damsels also chromis you will have to carefully choose fish if you plan to have shrimps or the shrimps may end up as an expensive meal